The invention resides in the field of sheet material processing including the graphic arts, paper making, and other fields which require the feeding or continuous, high-speed movement of sheet material. Most typically, the invention is applicable in a printing operation in which paper or other sheet material formed from fibrous particles is continuously fed and printed in a printing press.
In a continuous printing operation the paper is pulled by gripper bars along a circuitous path between and over cylinders which print the paper and deliver it in the conventional manner. The wetting (printing or coating) of one side of the paper and the curved path of travel of the sheet over the impression and other cylinders causes the sheet to curl about an axis spaced from the paper on the dry or unprinted side. In other words, the wet or printed side of the material is the convex side of the curl, and the dry or unprinted side is the concave side.
The curling of the paper causes problems of handling as the sheet is processed in the press and causes offset or print transfer problems at the delivery pile. The wet ink from the curled sheet is transferred to the unprinted side of the adjacent sheet above it. This problem does not occur when sheets are delivered flat due to the uniform floating of the upper sheet on the lower sheet.
Sheets delivered in a curled condition have a tendency to come to rest with their edges out of register with the edges of the adjacent sheets, thereby requiring increased shuffling or "jogging" of the sheets in the delivery pile to obtain a neat, registered stack. Thus, both the offset problem and need for increased "jogging" are caused by curled sheets and require slowing or stopping of the press for corrective action, which raises printing costs.
An additional shortcoming in the conventional printing operation is that the sheet material is pulled along a path by gripper bars which grip the leading edge of the sheet only, and the trailing edge is free and uncontrolled. Thus, there is no control or tension on the sheet, and operations such as slitting and trimming during the printing operation are difficult.
Prior art solutions to the curling problem have included bars, rollers or cylinders which engage the printed or wet side of the paper as the paper is pulled and bent around the bar, etc. While this method and apparatus decurls the sheet, it also smears the ink or other coating unless the printed or treated side is dry, a condition difficult or practically impossible to attain while the sheet is traveling through the press. Moreover, this prior art solution is not readily adjustable to compensate for variations in the speed of travel of the sheet material; variations in the properties, including thickness, of the sheet material; and variations in other conditions.